1. Field Of The Invention
This invention relates to the field of valving arrangements and more particularly, to the field of valving arrangements for negative pressure ventilators and methods for operating them.
2. Discussion Of The Background.
Negative pressure ventilators are widely used to assist patients who cannot breathe on their own or who simply need part-time or full-time assistance in breathing. In many cases, patients only use such negative pressure ventilators at night while they sleep to rest their chest muscles and diaphragm so they can then go through the day unaided.
Negative pressure ventilators essentially can do the work of breathing for the patient. In one common type of ventilator, a shell is placed over the front of the patient's chest. The shell is held in place on the patient by straps with the perimeter edge of the shell substantially conforming to the patient's body to form a seal. The main portion of the shell is spaced slightly away from or above the chest to form a chamber; and, a source of negative pressure such as a turbine fan or other blower is connected to the shell and operated to periodically draw or suck air out of the chamber between the shell and the chest. This action creates a negative pressure in the chamber above the chest (i.e., a pressure slightly less than ambient atmospheric pressure) causing the chest to be expanded upwardly into the chamber and the lungs to be filled with air.
More specifically, to take a breath, a person's chest is normally expanded by his or her own chest muscles and diaphragm to draw in the breath. In doing so, the expansion of the chest actually creates an area of pressure less than atmospheric in the chest cavity itself. The ambient air which is then at a relatively higher pressure simply flows into the lower pressure area of the expanded chest cavity filling the lungs with air. With a negative pressure ventilator, a slightly different principle is used wherein the ventilator does this work of breathing in place of the patient's own chest muscles and diaphragm. In doing so, the ventilator creates a volume of negative pressure (i.e., slightly less than atmospheric) outside of the patient's chest in the chamber between the patient's chest and the shell. The ambient air which is then at a relatively higher pressure seeks this volume of the chamber of negative or lower pressure and in doing so, flows into the patient's mouth and/or nose expanding his chest into the chamber and filling his lungs with air. The negative pressure in the chamber is thereafter reduced or eliminated wherein the patient's expanded chest falls essentially under its own weight to expel the breath from the lungs. This process is subsequently repeated at regular intervals (e.g., twelve times per minute) to simulate normal breathing.
In more sophisticated models of negative pressure ventilators, the exhalation of the breath from the patient's lungs can be aided by supplying a slight positive pressure to the chamber between the shell and the patient's chest. This positive pressure, in turn, assists in collapsing the patient's chest and forcing the air out of the patient's lungs. In most cases, the source of such positive pressure is simply the exhaust of the same turbine fan or blower that is being used to create the negative pressure in the chamber. The design problem then becomes how to connect and control the various flow paths to and from the ventilator in a precise and concise manner to alternately deliver such negative and positive pressures to the chamber between the patient's chest and the shell. The system should also preferably have a dead or neutral position in which neither positive nor negative air is delivered to the user. Further, the periodic delivery or flow of air must preferably be done without connecting the negative and positive sides of the turbine fan or blower at the same time to the chamber between the patient's chest and the shell. Otherwise, the opposing pressures would work against one another trying both to inflate and deflate the patient's lungs at the same time.
Such periodic delivery or flow of negative and positive pressure air into and out of the chamber between the shell and the patient's chest is commonly controlled by any number of valving arrangements. However, all such known ones involve at least two and usually four or even more distinct and separately operated valves or bleeders. This not only adds complexity and bulk to the ventilator but also can make its operation difficult to set and adjust. Further, it may make the inclusion of such desirable operating options as sigh features difficult to do.
With this in mind, the valving arrangement of the present invention was developed. With it, the periodic delivery or flow of negative and positive pressure air into and out of the ventilator and shell can be precisely controlled and adjusted by the operation of a single, one-piece valve member.